External evidence
"Our Father which art in heaven, Hallowed be thy name. Thy kingdom come. Thy will be done in earth, as it is in heaven. Give us this day our daily bread. And forgive us our debts, as we forgive our debtors. And lead us not into temptation, but deliver us from evil: For thine is the kingdom, and the power, and the glory, for ever. Amen." (Matthew 6:9-13b, KJV)
The underlined portion above is called the doxology of the Lord's prayer, or the ending to the Protestant version of the Lord's prayer. Roman Catholic and modern Bible versions of the prayer do not have this ending. This powerful doxology has been falsely characterized as a late addition as it is not found in the two earliest Greek witnesses of Matthew 6:13 - Sinaiticus and Vaticanus both from the 4th century. Yet it is found in the third earliest Greek witness of Matthew 6:13, Codex Washingtonensis from the 4th to 5th century. Hence a manuscript testifying for the doxology is preceded by only two adverse manuscripts, and that by just one or a half century. The doxology exists in the majority of Byzantine manuscripts (Nestle-Aland: Novum Testamentum Graece, 27th revised edition (2006)).
Codex Washingtonensis
Codex Washingtonensis is housed at the Freer Gallery, Sackler Museum, Smithsonian Institution, Washington D.C. and the passage of the Lord's Prayer can be viewed at the website of The Center for the Study of New Testament Manuscripts: http://www.csntm.org/Manuscript/View/GA_032 . Since uncials can be difficult to read, a photograph of the leaf containing the Lord's Prayer has been reproduced here (for non-profit educational fair use purposes) with the prayer underlined and the doxology in particular underlined in blue. The words of the Lord's Prayer in Codex Washingtonensis match the words of the Textus Receptus word for word, letter by letter for the most part.
Patristic Evidence
Some early Church fathers, such as Origen, Tertullian and Cyprian, omit the doxology, proving that some or perhaps many early Christians did not accept the doxology. However, these early omissions do not prove that the doxology was invented at a later time. Roman Catholics throughout history have omitted the doxology even during times when the doxology was prevalent in manuscripts or printed editions of the Gospel of Matthew. The Roman Catholic rejection of the doxology has had more to do with tradition rather than conclusions based on contemporaneous manuscript evidence. Likewise, the omissions by some early Christians could have been due to an early tradition rather than the absence of the doxology in their contemporaneous manuscripts.
We know that the doxology is very ancient because John Chrysostom (347–407) expounded the doxology in his homily, Homily 19 on St. Matthew, at paragraph 10 [Download a PDF of the Lord's Prayer portion of the Homily HERE]. He treats the words as those of Christ himself and says nothing of the alleged spuriousness of the words. This 4th century Archbishop of Constantinople would have had manuscripts that were earlier than what we have today. The earliest variant of the doxology appears in the Didache, an anonymous late first century treatise (click here to read it in Greek). It reads, "ὅτι σοῦ ἐστιν ἡ δύναμις καὶ ἡ δόξα εἰς τοὺς αἰῶνας", thereby omitting "η βασιλεια και" and "αμην". Some critics claim that this incomplete variant of the doxology proves the evolution of a spurious doxology. Such a conclusion is unwarranted, however, because the Didache is a slightly different version of the prayer to begin with. Instead of "τοις ουρανοις" (literally, "the heavens") the Didache has "τῷ οὐρανῷ" ("the heaven") - this change suggests that an editor converted the Hebraic expression of a plural heavens, as in שמים (shamayim), to the Greek idea of heaven as a singular location of God's dwelling-place. Also, instead of "τα οφειληματα ημων" ("our debts") the Didache has "τὴν ὀφειλη" ("debt"). The fact is that the Lord's prayer in the Didache has the doxology, albeit as an erroneous variant.
Also noteworthy is that both Vaticanus and Sinaiticus were deliberately kept hidden due to the fact that they contained hundreds, if not thousands of redacted writings that had originally matched other MS that were more recently produced.
It is a commonly held belief that they both were among the 50 Codices prepared by Origen, a known gnostic.
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(That, of course is a "King James Version Only" kind of web site, and their unorthodox opinions are not shared by most scripture scholars today, including those scholars at "Bible Hub".)